Data transfer errors are a major concern within any communication system. For example, efficient detection of these errors is quite challenging, because error checks cost the system resources, such as bandwidth. As a result, a fewest number of error detection mechanisms are preferred in any given system.
The problem is, however, that when too many bit errors corrupt a data packet, the corrupted data is preferably mitigated instead of using the corrupted packet. At the same time, proper mitigation of the corrupted data packet becomes more difficult when fewer error detection mechanisms are implemented. In particular, when an error is found during a data error check, the entire data block is typically found to be corrupted, even though the error is generated by a small portion of the entire data block. Thus, the more data error checks that are implemented, which divide the data packets into smaller data blocks, the more likely that mitigation of corrupted data can be performed more smoothly, because uncorrupted data are less likely to be tossed out in the system. This benefit, however, is gained at the cost of system resources. As a result, there is a tradeoff between accuracy and system resources.
This is especially problematic in cellular communication systems where multiple speech frames are delivered within a single data packet, and voice quality is a key concern in such a system. For example, mitigation in existing systems, such as iDEN, employs a single Cyclic Redundancy Check (“CRC”) for each data packet to detect data errors during transmission. The detected errors are then mitigated to reduce their effect. Mitigation in the iDEN® system generally takes one of two forms of either repetition or muting. Repetition uses the last known good packet, while muting generates so-called “comfort noise” instead of the corrupted packet. In cases where repetition is preferred over muting, repetition, unfortunately, is usually only good for one or two corrupted packets, after which the repeated audio packet becomes stale and its use constitutes an unacceptable audio artifact. In other words, when the repeated audio is too remote from its original location in the sequence, the quality of the overall audio-stream is more degraded when outputted to the user.
Since each audio data packet contains multiple speech frames, an error detection mechanism is ideally implemented for each of the speech frames to guarantee that all the corrupted speech frames can be accounted for. This method, however, requires too much bandwidth, which is generally limited in a cellular system. Because the current iDEN system uses only one CRC for each data packet to detect errors due to bandwidth constraints, some of the data may be valid, despite the error detection from the CRC. Uncorrupted data, as a result, are being unnecessarily replaced. Moreover, with the use of a single CRC for each data packet, the system is limited to a single-tiered mitigation approach.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.